Control of plasma



Dec. 7, 1965 K. c. RICHARDS 3,222,568

CONTROL 0F PLASMA Filed Sept. 18, 1963 2 Sheets-Sheet l Dec. 7, 1965 Filed Sept. 18, 1963 2 Sheets-Sheet 2 MMM o 20 40 eo 8o /aa /zo /40 /a /30 zoo ,22o 24a ,26o 26a United States Patent O 3,222,568 CONTRQL F PLASMA Keith C. Richards, Cleveland, Ohio, assignor to Owens- Illinois Glass Company, a corporation of Ohio Filed Sept. 18, 1963, Ser. No. 309,746 6 Claims. (Cl. 315-111) This invention relates to the generation of plasma and particularly to th-e problem of controlling plasma.

In the utilization of a plasma flame or jet, it is desirable to maintain the heat content and ionization level of the plasma jet substantially constant. It has heretofore been suggested that this might be achieved indirectly by measuring the temperature of the plasma jet and controlling the generation of the plasma jet in response to variations in temperature. However, such a control has not been found to produce proper control.

It is an object of this invention to provide a method and apparatus for controlling the generation of a plasma jet and maintaining the heat content and ionization level ofthe plasma jet substantially constant.

It is a further object of the invention to provide an apparatus which is relatively simple and low in cost.

In the drawings:

FIG. 1 is a diagrammatic view of a plasma generator system embodying the invention.

FIG. 2 is a part sectional view of a plasma generator embodying the invention.

FIG. 3 is a set of curves of plasma current versus probe current in an apparatus embodying the invention.

FIG. 4 is a schematic, perspective view of the electrodes of FIG. 2.

Referring to FIG. 1, the invention comprises positioning a pair of oppositely disposed probes 1t), such as platinum electrodes, in a comparatively cool portion of the plasma jet that is generated by plasma generator 11. The electrodes are connected in a circuit including a constant source 12 of voltage. Variations in current ilow between the electrodes 10 are amplified by a direct current amplier 13 and utilized to activate a control system 14 which controls the application of electric power through lines 15, 16 to the plasma generator 11. The control unit utilized in the device 14 preferably comprises a saturable core reactor to which the amplified and reversed polarity current from the direct current amplifier 13 is applied.

If, for example, a reduction in current between the electrodes 10 is noted, the control unit and rectifier 14 causes an .increase in application of electric power to 11, in turn, increasing the generation of plasma and bringing the electric current between the electrodes 10 back to its original level. If the current between th-e electrodes 10 increases, the electric power applied to the plasma generator 11 is decreased to return the current between the electrodes 10 to its original value.

It can thus be seen that, in accordance with the invention, the current between the electrodes 10 is maintained substantially constant. Since the current between the electrodes will depend upon the ionization level of the plasma jet, it is thereby possible to directly control the ionization level of the plasma jet. Further, since the heat content of the plasma jet is a function of the ionization level, in accordance with the invention, the heat content of the plasma jet as well as the ionization level is being controlled.

It is thus possible in any specic application of the plasma jet to maintain the ionization level and heat content substantially constant. For example, in spheroidizing and crystal growth, the heat content is the major condition which causes variations in the product and it is possible with the present invention to maintain the heat content substantially constant and thus accurately control the major production parameter. In chemical synthesis,

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ionization is an important consideration and it is possible with the present invention to maintain ionization substantially constant or accurately control the ionization level.

For any particular application of the plasma jet system embodying the invention, the system must be calibrated. Thus, for any spheroidizing, crystal growth or chemical synthesis application, a set of calibration curves must be established, as presently described.

Referring to FIG. 2, a typical plasma generator 20 comprises that known as an F-4() plasma llame torch made by Thermal Dynamics Corporation of Lebanon, New Hampshire. Such a generator 20 consists of a main brass body in which a nylon body 21 is mounted and supports a cathode 22 which may comprise 2 percent thoriated tungsten. A cooling water inlet and cathode connection 23 is provided on the body 20 and a cooling water outlet and anode connection 24 is also provided on the body 20. The plasma gas inlet 25 Iprovides gas through a passage 26 adjacent a ceramic gas ring 27 to the area of the cathode 22. A nozzle anode 28 is provided adjacent and surrounding the plasma jet that is formed. This anode 28 carries an internal copper sleeve 29. The generator 20 includes inlets 30, 31 adjacent the periphery of the plasma jet through which various ingredients that are to be acted upon by the plasma jet can be introduced. In accordance with the invention, the electrodes 101 are provided adjacent the periphery in a relatively cool portion of the plasma jet.

As shown in FIG. 4, the electrodes 101 are preferably rectangular in shape and have at surfaces 32.

The aforementioned description of the plasma generator shown in FIG. 2 is conventional and well known in the art, except for the electrodes 101.

In a typical example of a plasma generator system embodying the invention, a pair of electrodes 101 having a dimension of one-eighth inch by one-quarter inch were positioned a half inch in front -of the nozzle with a gap between the electrodes of about three-eighths inch. The plasma generator had a voltage of 160 volts applied thereto and argon gas supplied at varying rates. FIGURE 3 is a plot of curves obtained by maintaining the plasma generator at 16() Volts and systematically varying argon ow rate from cubic feet per hour to 150 cubic feet per hour.

It can be appreciated tha-t for any particular operation wherein added materials are being fed through inlets 31 to the plasma generator, a new set of calibration curves will have to be developed which will be applicable to the new conditions.

I claim: 1. In a plasma generator wherein a plasma jet is formed and `maintained between a cathode and an anode by the application of electric .power to the cathode and anode, a control device comprising a pair of oppositely disposed electrodes adapted to be placed in a relatively cool portion of the plasma jet,

means for measuring variations in conductivity across said electrodes when the electrodes are in position,

and means for controlling the current ow between the cathode and anode of the plasma generator in response to variations in conductivity between the electrodes to maintain the current between the electrodes substantially constant and, in turn, the ionization level and heat content of the plasma jet constant.

2. In a plasma generator wherein a plasma jet is formed and maintained between a cathode and an anode by the application of electric power to the cathode and anode, a control device comprising a pair of oppositely disposed electrodes adapted to be placed in a relatively cool portion of the plasmas jet, means for applying a substantially constant voltage across said electrodes,

means for measuring variations in conductivity across said electrodes when the electrodes are in position, and means for controlling the current flow between the cathode and anode of the plasma generator in response to variations in conductivity between the electrodes to maintain the current between the electrodes substantially constant, and in turn, the ionization level and heat content of the plasma jet constant. 3. In a plasma generator wherein a plasma jet is formed and maintained between a cathode and an anode by the application of electric power to the cathode and anode, a control device comprising a pair of oppositely disposed electrodes adapted to be placed in a relatively cool portion of the plasma jet, means for applying a substantially constant voltage across said electrodes, means for measuring variations in conductivity across said electrodes when the electrodes are in position, means for increasing the current flow between the cathode and anode of the plasma generator in response to reductions in the conductivity of the plasma jet and for decreasing the current flow between the cathode and anode in response to increases in the conductivity of the plasma jet in order to maintain the conductivity of the plasma jet substantially constant and, in turn, the ionization level and heat content of the plasma jet constant. 4. The combination set forth in claim 3 wherein said last-mentioned means comprises a saturable core reactor. 5. In a plasma generator wherein a plasma jet is formed and maintained between a cathode and an anode by the application of electric power to the cathode and anode, a control device comprising a pair of oppositely disposed electrodes adapted to be placed in a relatively cool portion of the plasma jet,

means for measuring variations in conductivity across said electrodes when the electrodes are in position, means for increasing the current ilow between the cathode and anode of the plasmas generator in response 5 to reductions in the conductivity of the plasma jet and for decreasing the current flow between the cathode and anode in response to increases in the conductivity of the plasma jet in order to maintain the conductivity of the plasma jet substantially constant and, in turn, the ionization level and heat content of the plasma jet constant.

6. In a plasma generator wherein a plasma jet is formed and maintained between a cathode and an anode by the application of electric power to the cathode and anode, a control device comprising means for applying a substantially constant voltage across a relatively cool portion of the plasma jet, means for measuring variations in conductivity across said relatively cool portion of the plasma jet,

and means for controlling the current tlow between the cathode and anode of the plasma generator in response to variations in conductivity to maintain the current across the relatively cool portion of the plasma jet substantially constant, and, in turn, the ionization level and heat content of the plasma jet constant.

References Cited by the Examiner UNITED STATES PATENTS GEORGE N. WESTBY, Primary Examiner. 

1. IN A PLASMA GENERATOR WHEREIN A PLASMA JET IS FORMED AND MAINTAINED BETWEEN A CATHODE AND A ANODE BY THE APPLICATION OF ELECTRIC POWER TO THE CATHODE AND ANODE, A CONTROL DEVICE COMPRISING A PAIR OF OPPOSITELY DISPOSED ELECTRODES ADAPTED TO BE PLACED IN A RELATIVELY COOL PORTION OF THE PLASMA JET, MEANS FOR MEASURING VARIATIONS IN CONDUCTRIVITY ACROSS SAID ELECTRODES WHEN THE ELECTRODES ARE IN POSITION, AND MEANS FOR CONTROLLING THE CURRENT FLOW BETWEEN THE CATHODE AND ANODE OF THE PLASMA GENERATOR IN RESPONSE TO VARIATIONS IN CONDUCTIVITY BETWEEN THE ELECTRODES TO MAINTAIN THE CURRENT BETWEEN THE ELECTRODES SUBSTANTIALLY CONTANT AND, IN TURM, THE IONIZATION LEVEL AND HEAT CONTENT OF THE PLASMA JET CONSTANT. 